Flow rate control mechanisms are used in a variety of flow systems as a means for controlling the amount of fluid, gas or liquid, traveling through the system. In large-scale processing systems, for example, flow control may be used to affect chemical reactions by ensuring that proper feed stocks, such as catalysts and reacting agents, enter a processing unit at a desired rate of flow. Additionally, flow control mechanisms may be used to regulate flow rates in systems such as ventilators and respirators where, for example, it may be desirable to maintain a sufficient flow of breathable air or provide sufficient anesthetizing gas to a patient in preparation for surgery.
Typically, flow rate control occurs through the use of control circuitry responsive to measurements obtained from carefully placed flow sensors. One such flow sensor is a thermal anemometer with a conductive wire extending radially across a flow channel and known as a hot-wire anemometer. These anemometers are connected to constant current sources, which cause the temperature of the wire to increase proportionally with an increase in current. In operation, as a fluid flows through the flow channel and, thus, past the anemometer, the wire cools due to convection effects. This cooling affects the resistance of the wire, which is measured and used to derive the flow rate of the fluid. Another form of thermal anemometer flow sensor is a microstructure sensor, either a micro bridge, micro-membrane, or micro-brick, disposed at a wall of a flow channel. In this form, the sensors ostensibly measure the flow rate by sampling the fluid along the wall of the flow channel. In either application, the thermal anemometer flow sensor can be disposed in the flow channel for measuring rate of flow.
There are numerous drawbacks to these and other known flow sensors. The flow restriction mechanisms arranged inside a flow channel can create a pressure drop across the sampling ports that facilitate fluid flow into a sensing channel. This pressure drop, or pressure differential, is dependent on restrictor geometry and ingresses with flow rate. Furthermore, the fluid in the flow channel may possess an increasingly turbulent flow as the flow rate of fluid increases (i.e., an increasing non-uniform pressure and velocity across a given plane orthogonal to the direction of flow).
When combining flow sensors with other sensing measurands, it is critical not to create turbulent flow, because such flow can in turn decrease the accuracy of the flow measurement. Therefore, to overcome the forgoing shortcomings, it is desirable to provide for a suitable packing method and/or system for measuring multiple measurands including bi-directional flow.